"This dress is easy to get dirty."
Translation:Gaun ini mudah kotor.
They are not using the verb "to get/to be" (menjadi) in the answer. So it kind of translates as: This gown is easily dirty?
Yes, that's correct.
"This gown is easily dirty?" ==> I think that's a better EN translation of the ID sentence.
The current EN translation still needs to be changed, see this sentence discussion:
Adverbs can modify verbs, or copulae, or other adverbs. While the copula isn't explicit in this sentence, it's still implied, so it can be modified with an adverb. The same thing happens with English informal zero-copula sentences like Toni Basil's, "Oh Mickey you so fine"
The important thing to take away here is that mudah can be used as an adverb.